String Theory, Quantum Gravity and Black Holes (Or, Are We Holograms?)

The conversation begins with Brian Green introducing the topic of unifying gravity and quantum mechanics, a long-standing challenge in theoretical physics. He is joined by Juan Maldacena, a prominent figure in the field, to discuss the difficulties and advancements in this area. They delve into the significance of understanding these theories, particularly in the context of black holes and the early universe, and touch upon the potential of string theory as a solution.

The discussion shifts to the role of black holes in the quest for quantum gravity. Maldacena and Green explore the unique properties of black holes, where spacetime collapses and both quantum mechanics and gravity become crucial. They discuss the implications of understanding the interior of black holes for the broader goal of unifying physical theories and the potential insights gained from the Event Horizon Telescope's images of black holes.

Green and Maldacena reflect on the historical development of general relativity and quantum mechanics, highlighting Einstein's skepticism about certain aspects of the theories he helped establish. They discuss the probabilistic nature of quantum mechanics and Einstein's discomfort with it, as well as the evolution of physicists' acceptance of these theories over time. Maldacena shares his own experiences with quantum mechanics and the challenges it presents.

The conversation continues with a look at how the understanding and importance of entanglement in quantum mechanics have evolved over time. Maldacena notes the shift in focus from atomic and subatomic particles to complex quantum materials and computers, where entanglement plays a fundamental role. They discuss the historical recognition of entanglement's significance and its current relevance in quantum computing and other applications.

Green and Maldacena delve into the technical challenges of merging general relativity and quantum mechanics. They explain how the two theories apply at different scales and the difficulties that arise when trying to apply them simultaneously. Maldacena discusses the theoretical issues when extrapolating these theories to the extremes of time, such as the beginning of the universe or the interior of black holes, and the desire for a single, unifying theory.

The discussion touches on the risks and rewards of theoretical pursuits in physics, particularly in the context of string theory. Maldacena reflects on the challenges of making predictions at vastly smaller scales than can be experimentally verified. Green asks Maldacena about his perspective on the theoretical leap string theory requires and whether it's too early in the history of science to pursue such a unification. Maldacena expresses optimism, given the constraints and consistency found within string theory.

Maldacena provides a historical overview of string theory, from its initial development aiming to understand strong interactions to its emergence as a leading candidate for a theory of everything. He discusses the excitement around the discovery that string theory naturally includes a particle with the properties of gravity, the graviton. The conversation highlights the evolution of string theory, its challenges, and the hope that it may provide a consistent framework for unifying all fundamental forces.

Green and Maldacena discuss the extra dimensions predicted by string theory and the implications of these dimensions on the laws of physics. They explore the concept of compactifying these dimensions and the potential for different compactifications to lead to different physical realities. Maldacena shares his initial disappointment with the proliferation of possible compactifications but also acknowledges the opportunities they present for connecting string theory to observable phenomena.

The conversation turns to the holographic principle and its implications for understanding black hole entropy. Maldacena explains how the entropy of a black hole is proportional to its surface area rather than its volume, suggesting a fundamental shift in how we think about spacetime and gravity. They discuss the holographic principle's roots in the work of 't Hooft and Susskind and how it has been further developed and applied in the context of the AdS/CFT correspondence.

Maldacena and Green delve deeper into the AdS/CFT correspondence, a groundbreaking development in string theory that provides a way to understand the dynamics of black holes and gravity through a quantum field theory without gravity. They discuss the implications of this correspondence, the insights it offers into the nature of spacetime and gravity, and its potential for experimental verification through the creation of strongly interacting quantum systems.

In the final part of the conversation, Maldacena and Green reflect on the progress made in string theory and quantum gravity over the past 30 years and express optimism for the future. They discuss the potential for experimental predictions and the ongoing search for a more comprehensive understanding of the nature of spacetime, gravity, and matter. Maldacena shares his thoughts on the possibility of creating small, artificial universes in a lab to test gravitational theories and the exciting new ideas emerging in the field.